At present, the development of high-power microwave electronics is aimed at maximizing the pulse and continuous (or average) power output, as well as the pulse energy. The power output of microwave devices, and more specifically magnetron-type microwave devices, is limited by the properties of the materials of the cathode, anode and dielectric output window, their ability to withstand and dissipate electrical and thermal loads, as well as by the electronic efficiency.
These limitations are obviated as follows: by increasing the electronic efficiency, by using special materials with improved emission properties for cathodes, materials featuring high electrical and heat conductance for anodes and cathodes, materials with maximum tolerance to thermal loads for anodes, and materials with low dielectric loss and high microwave carrying capacity for output windows, etc.
However, the properties of materials have certain physical limitations as far as maximum permissible thermal and electrical loads are concerned, therefore further increases in the microwave generation power are possible only through enhancing the electronic efficiency and enlarging the working surfaces of the electrodes--both the anode and cathode. This possibility of increasing power is illustrated by the following formula for the limiting average (or continuous) power output of a microwave magnetron-type device: EQU P=qS(.eta..sub.e /1-.eta..sub.e), (1)
where P is the limiting average (or continuous) microwave power output;
q is the maximum permissible specific load on the anode; PA1 S is the working surface of the anode; PA1 .eta..sub.e is the electronic efficiency. PA1 .eta. is the electronic efficiency of a microwave magnetron-type device, PA1 .eta..sub.e is the electronic efficiency of energy transformation, PA1 .eta..sub.n is the electronic efficiency of individual cavities of the retarding system of the device, PA1 Q.sub.n is the loaded Q factor of the retarding system of the device, PA1 Q.sub.o is the intrinsic Q factor of the retarding system cavities. PA1 d.sub.1 is the distance between the vane ends and the outer surface of the anode assembly, PA1 d.sub.2 is the distance between the vane ends and the duct wall adjacent to the outer surface of the anode assembly, PA1 d.sub.3 is the distance between the vane ends and the wall of the inductive portion of the cavities, remotests from the vane ends, PA1 d.sub.4 is the distance between the vane ends and the opposite duct wall, PA1 d.sub.5 is the distance between the vane ends and the window wall on the side of the vane ends, PA1 h.sub.1 is the height of the duct between the two stages of the retarding system, PA1 h.sub.2 is the distance between adjacent stages of the retarding system.
Since the electronic efficiency of microwave magnetron-type devices may be high (90% and higher), with the maximum attainable electronic efficiency the only parameters that can be varied to further improve the microwave power output are the surface of the anode (as well as the cathode) and the maximum permissible specific load on the anode. When the maximum specific loads are attained, only one variable parameter is left--the anode surface.
In microwave devices of the magnetron type, the working surfaces of the electrodes, cathode and anode, are enlarged by increasing their radial and axial dimensions (if the anode assembly or block is cylindrical in shape). Larger working surfaces mean greater mass of the device. Low-power microwave magnetron-type devices are rather compact, and the ratio of mass M of the device to its power P is sufficiently low (M/P.about.0.5 to 1 kg/kW) and meets such design and performance criteria as low metal requirements, stability to mechanical damage in manufacture and handling, low cost, etc.
In the case of high- and, especially, extremely high-power microwave magnetron-type devices, the problem of weight and size attains primary importance and, in some instances, when there are such physical limitations as yield of the material, which does not provide for the desired stability of shape in heavy devices, the weight and size are the determining factors in creating a microwave device with a required power output.
Despite the fact that the ratio of mass to microwave power output (M/P) in such devices is approximately the same as in low-power devices, and in some cases even somewhat lower, the absolute value of mass increases with the working surface of the anode assembly.
At the same time, increasing the size and weight (mass) of a microwave magnetron-type device with a view to enhancing its power output means to increase the size and weight of the magnets creating the magnetic field in the device. All this leads to serious difficulties in developing devices with high power output.
The problem is further aggravated when the wavelength of the generated microwaves is increased. The bulk of the anode assembly in microwave magnetron-type devices containing a multicavity retarding system is occupied by cavities whose dimensions determine the length of the generated microwaves. In order to ensure maximum electronic efficiencies of the device as a whole (.eta.) and individual cavities (.eta..sub.n), the intrinsic Q factor (Q.sub.o) of the latter must be as high as possible, which can be inferred from formula: EQU .eta.=.eta..sub.e .multidot..eta..sub.n =.eta..sub.e (1-Q.sub.n /Q.sub.o) (2)
where
Since the intrinsic Q factor Q.sub.o of the cavities is, in turn, directly dependent on the volume of their inductive part, the size and weight (mass) of a microwave device is largely dependent on the construction of the inductive part of the cavities.
It should be pointed out that the intrinsic Q factor of the cavities also determines other characteristics of a microwave device, particularly the stability of its operation.
The fabrication of microwave magnetron-type devices also involve manufacturing and economical problems. Namely, each time when a device generating microwave power at a given frequency has to be made, it is necessary to fabricate a rather complex tool, e.g. a punch, for imparting the cavities of the retarding system a shape corresponding to that frequency. This considerably complicates the process of manufacturing new devices and increases its cost, particularly when the operation of the device must be extremely accurate at a strictly defined resonance frequency (or within a frequency band) and when a broad frequency band has to be covered by several almost identical devices of standardized design.
Known in the art is a microwave magnetron-type device comprising an anode assembly with a retarding system made up of cavities with Z-shaped segments or vanes, as well as straps. The straps are electrically associated with respective vanes of cavities of the same polarity in the pi-mode (cf. U.S. Pat. No. 2,953,715; Cl. 315-39.75; Sept. 20, 1960).
In this device, the cavity vanes being Z-shaped does not permit substantially increasing the axial dimensions of the anode assembly in order to enhance the power output because the dimensions of the cavity vanes along the height of the anode assembly or anode block exceed the axial dimensions of the working part of the anode assembly.
Thus, in the prior art design, the reduction of the diameter of the cylindrical anode assembly results in a greater height thereof without increasing the anode surface, hence, the generating power. Therefore, no gain is achieved by reducing the size and weight.
In addition, in the above microwave magnetron-type device, the anode assembly and its retarding system cannot be made strong and stable in shape, particularly in the case of a large anode assembly. This is due to the fact that the point of attachment of a vane to the anode assembly is relatively far from the vane's center of gravity, and without additional support the vanes may bend.
Also known is a microwave magnetron-type device comprising an anode assembly in the form of a multistage two-dimensional periodic retarding system, including cavities with vanes each having a lumped inductance portion defined by the vane sidewalls near the vane bases or roots, and straps. The straps are arranged on each stage of the retarding system and pass through windows made in each of the vanes. The device also comprises a cathode arranged spaced a distance from free ends of the vane.
In this microwave device, the anode assembly is cylindrical in shape, the height of the cylinder is greater than a quarter of the generated wavelength .lambda., while the distance between adjacent stages of the retarding system, i.e. the distance between adjacent double straps, does not exceed .lambda./6, which provides for some increase in the generated power level.
However, this entails not only a larger size of the device but also a greater mass thereof. The larger size and mass are primarily due to the necessity to increase not only the axial dimensions of the cylindrical anode block or assembly but also its diameter. This is caused by the fact that the larger axial dimensions of the anode assembly, i.e. its height, result in lower inductance L thereof, due to the inductance being inversely proportional to the anode height.
The presence of straps, increasing their number and the total capacitance with the height of the anode assembly, result in some increase in the wavelength .lambda. of the generated microwaves, but the efficiency of the device drops because of the lower intrinsic Q factor Q.sub.o of the retarding system.
This calls for increasing the inductance of the cavities and decreasing the total capacitance with a constant wavelength of the retarding system of the anode assembly by increasing the volume of the cavities in their inductive portion. Normally, this can be done in a cylindrical anode assembly by increasing the transverse dimensions of the cavities, which results in a greater diameter of the anode assembly as a whole.
The same applies to the case where the wavelength .lambda. of the generated microwaves has to be increased. To this end, either the number of straps and the height of the anode assembly are increased, or the radial dimensions of the cavities and the anode assembly, or both.
Thus, the above construction of a microwave magnetron-type device does not permit reducing the mass and weight of the latter, both when the working surface of the anode assembly is extended to increase the generated microwave power output of the device and when the wavelength is increased to extend the application of such microwave devices to the long-wave region of the microwave range.
Besides, the above design of a microwave magnetron-type device does not permit standardizing devices intended to perform similar functions over a broad microwave frequency range.